Proteins are dynamic molecular machines, undergoing motions on a wide range of time scales. Although there is considerable evidence both from theory and experiment that many enzymes are inherently flexible, fundamental questions about the relationship between protein dynamics and enzyme catalysis remain unanswered. Are protein motions coupled to the chemical transformation, or are they involved primarily in controlling the flux of substrate, products, or cofactors? What role do protein motion play in progression from the preorganized ground state structure to an active site configuration that facilitates the chemical reaction? What is the time scale of active site conformational changes required for catalysis? How is the energy landscape of the enzyme modulated during the catalytic cycle and how is it shaped during evolution? Are there species-related differences in protein dynamics and available conformational substates that might potentially be exploited for development of highly selective drugs that better discriminate between enzymes from humans and pathogens? These issues will be addressed using state-of-the-art NMR methods and multi-conformer room temperature X-ray crystallography to elucidate the dynamic properties of an exceptionally well- characterized enzyme, dihydrofolate reductase (DHFR). DHFR is the target for anti-folate drugs such as the anticancer agent methotrexate and the antibacterials trimethoprim and iclaprim. The proposed research will lead to new understanding of the intrinsic molecular dynamics of this important enzyme and how its motions are modulated by interaction with substrate, cofactor, and products at various stages in the catalytic cycle. It will also provide novel insights into the role of evolution in shaping the energy landscapes of E. coli and human DHFR and will advance our understanding of the relationship between protein dynamics and catalytic function.